Toyota did a study and found that pretty much no one who owns a PHEV ever plugs it in. I know you qualified with with "proper battery management", but the reality is most people will be lazy and cannot be bothered.
Er, yeah, but that's an entirely different problem than the one I was commenting on. If you never plug your PHEV in your battery will last forever since it will take ages for the capacity to drop to where it no longer functions as a sink for regen braking.